The present invention relates to a closed cycle refrigeration system utilizing a remote condenser and constructed so as to improve the efficiency of operation of the system and reduce the power consumption.
In the basic construction and operation of a closed cycle refrigeration system, gaseous refrigerant, e.g., freon, is compressed to a high temperature and pressure. The compressed gas is passed to a condenser where it is condensed to a liquid phase. The pressure within the condenser is maintained high enough that the condensing temperature is higher than the ambient air temperature. The liquid refrigerant may be temporarily stored in a receiver before being passed, through a metering device to reduce the liquid refrigerant pressure, to an evaporator located within a display case. As the liquid passes through the evaporator, it extracts heat from the display case and undergoes a phase change to the gaseous state. This low pressure gaseous refrigerant is supplied to the input side of the compressor where it is heated and compressed to a high pressure and the cycle is continued.
Traditionally, the condenser was operated at a preselected design temperature level. The design temperature for the condenser was generally determined as a function of the highers ambient temperature during a normal period of the warmest season in a particular area. The condenser was operated so as to condense the gaseous refrigerant at a temperature of at least 10.degree. F. above this design temperature. Consequently, if the design temperature was 90.degree. F., then the condenser temperature was set at 100.degree. F.
Recognizing that the design temperature was only likely to occur a few days in a year, and then only during the day and not at night, the refrigeration systems have been modified so that the condenser temperature followed the path of the ambient temperature while always remaining at least 10.degree. F. above the ambient temperature. Varying the condenser temperature to follow ambient conditions results in increased compressor capacity. The rule of thumb is that every 10.degree. F. drop in the condenser temperature increases the compressor capacity by about 6%. Thus, if the condenser temperature drops from 100.degree. to 75.degree., the compressor capacity will increase by about 15%. Simultaneously, the compressor consumption will be reduced, the compressor efficiency will increase, and the BTU/Watt of the compressor will increase. The combination effect is to increase compressor capacity and reduce power consumption, assuming constant refrigeration load. If the condenser temperature drops from 100.degree. to 75.degree., for example, consumption is reduced by almost 20%, assuming a constant refrigeration load.
During the operation of the refrigeration systems, it is necessary to regulate the pressure within the receiver in order to ensure proper operation of the evaporators. Such regulation has typically been provided by shunting hot gaseous refrigerant from the gas discharge line of the compressor directly into the receiver whenever the relative pressure of the receiver drops by more than a preselected pressure differential from the pressure in the gas discharge conduit. For such purposes, a check valve, typically set to respond to a pressure difference on the order of 20 or 30 psi, has been provided in the line between the gas discharge conduit and the receiver. Hence whenever the pressure within the receiver drops by more than 20 or 30 psi as compared to the pressure in the gas discharge conduit the check valve opens and allows the hot gas from the gas discharge line to flow directly into the receiver. Since the gaseous refrigerant in the gas discharge conduit is typically of a temperature level of approximately 200.degree. F., such gas acts as a significant heat source to the receiver, a situation which is generally undesirable.
During the refrigeration cycle, the refrigerant absorbs a substantial amount of heat during the evaporation stage, which heat is then dissipated by the condenser as a waste by-product of the refrigeration cycle. A technique for taking advantage of the heat to be dissipated by the hot gaseous refrigerant is the utilization of a heat recovery coil, such as shown in U.S. Pat. No. 4,123,914 issued Nov. 7, 1978, to Arthur Perez and Edward Bowman, and commonly assigned with the present invention. The disclosure of the Perez et al. '914 patent is incorporated herein by reference. Such a heat recovery coil allows for extraction of heat from the gaseous refrigerant flowing out of the compressor before entering the remote condenser. Such extracted heat then can be utilized for heating the interior of the building where the refrigeration system is employed.
Especially in recent years, much attention has been directed to improving the efficiency of such refrigeration systems. The prior art is replete with discussions of various techniques for attempting to improve the operation of a refrigeration system. In large installations, such as supermarkets, there are typically large numbers of refrigerated display cases and a plurality of compressors are used to satisfy the heavy refrigeration load under certain conditions, such as during the warmer portions of the year. The efficiency of the compressors is dependent upon the compression ratio of the discharge side of the compressor to the suction side of the compressor. Thus, by reducing the head pressure at the compressor discharge, the efficiency of the operation of the compressor can be increased. One such system, employing reduced head pressure to increase operating efficiency, is described in copending application Ser. No. 57,350, filed July 13, 1979, now U.S. Pat. No. 4,286,437 titled ENERGY SAVING REFRIGERATION SYSTEM and commonly assigned with the present invention; the disclosure of said Ser. No. 57,350, now U.S. Pat. No. 4,286,437 is hereby incorporated by reference as though fully set forth herein.
One of the features of the low head pressure systems, particularly including the one described in the aforesaid application Ser. No. 57,350, now U.S. Pat. No. 4,286,437 is that the system is designed to subcool liquid refrigerant in the remote condenser. Liquid subcooling will increase the efficiency of the system since the refrigerant will extract 15-25% more heat per pound circulated. The rule of thumb is that for every 10.degree. F. subcooling the system efficiency will increase by 5%. In substantially all commercial refrigeration systems, a receiver tank or surge tank is interposed between the condenser output and the liquid manifold supplying the evaporator coil. It has been found that, in systems employing a receiver tank, the refrigeration loses 10.degree. to 15.degree. F. of subcooling in passing through the receiver; that is, the temperature of the refrigerant in the receiver may rise 10.degree. to 15.degree. F. This results in a loss of efficiency since fewer BTU's of heat can be extracted from the air around the evaporator coils in the display case for each pound of refrigerant passing through the evaporator coil. One reason for this heat gain is that the receiver tank is generally located in the machinery room adjacent the compressor motors and related heat producing equipment. The temperature in the machinery room will usually be higher then the outside ambient temperature. Some of this heat will be absorbed by the refrigerant in the receiver and the temperature of the refrigerant will rise accordingly.
Some commercial refrigerating systems attempt to avoid the problem of receiver tank heat gain by using a surge tank; one such surge tank system is shown in U.S. Pat. No. 3,905,202 issued Sept. 16, 1975 to Donald F. Taft et al. In a surge tank type of system, condensed liquid refrigerant flows directly from the condenser output to the case evaporators. The surge tank stores excess liquid refrigerant to assure continued operation under varying ambient conditions which result in a variation in the condensing capacity of the condensers. It has been found that, especially during hot weather operations, the closed circuit system may "die" because the surge tank pressure may run 35 to 40 psig lower than the condenser pressure, resulting in liquid refrigerant logging in the receiver and not being passed to the evaporator. This problem is particularly prone to occur during periods of abnormally high ambient temperature; at such times, the rated design temperature of the condenser will be exceeded and the condenser will be unable to completely condense the refrigerant. The refrigerant will thus tend to collect and be condensed in the surge tank, creating a pressure drop upstream of the evaporators.
The present invention constitutes an improvement over prior art receiver tank and surge tank systems. The present invention incorporates a bypass conduit which permits subcooled liquid refrigerant to flow directly from the condenser to the evaporator coils under normal temperature conditions without first passing through the receiver tank.
In one embodiment of the present invention, the receiver tank is configured to have its input and output located at the bottom of the tank. The lower half of the tank is insulated to minimize heat transfer from the machine room to the liquid refrigerant in the bottom portion of the receiver tank. The upper half of the receiver tank is exposed to the machine room ambient, preferably equivalent to no lower than 65.degree. F. and no higher than 110.degree. F., which allows for boiling off of refrigerant from the liquid surface; this produces a corresponding pressure equivalent to 125 psig in the receiver tank.